1. Technical Field
The invention relates to atmospheric spectroscopy systems. More particularly, the invention relates to atmospheric spectroscopy systems designed to obtain measurements in atmospheric regions above areas to which access is difficult.
2. Description of the Prior Art
Systems capable of measuring the chemical composition of the Earth""s atmosphere have proven to be of great utility in a wide range of scientific research. Atmospheric spectroscopy is one commonly used technique for making such measurements. Atmospheric spectroscopy has, in part, enabled the study of increasing urban pollution, global warming, and the depletion of the ozone layer.
One difficulty faced in applying atmospheric spectroscopy is that of gaining access to the atmospheric region for which measurements are desired. One very direct approach is to transport the spectrometer to the desired location. An emitter and receiver pair, typically a laser and an optical transducer in close proximity, then take measurements for the point of interest.
An example of this technique is the Aircraft Laser Infrared Absorption Spectrometer (ALIAS) (http://laserweb.jpl.nasa.gov/alias.htm), a xe2x80x9chigh resolution four-channel scanning tunable diode laser spectrometer (3.4-8.0 xcexcm) which makes direct, simultaneous measurements of HCl, NO2, CH4, N2O, and CO . . . in the stratosphere and troposphere at sub-parts-per-billion sensitivities.xe2x80x9d
In another approach, measurements are made for a column of air above a ground-based instrument. In this case, radiation is emitted upwards, and the reflected or backscattered radiation is analyzed. One such system is in use at the NASA Light Detection and Ranging laboratory (http://tmf-web.jpl.nasa.gov/projects.html). xe2x80x9cThe systems are designed to make high-precision, long-term measurements to aid in the detection of atmospheric changes.xe2x80x9d More specifically, xe2x80x9ca Nd:YAG-based system is used for measurements from ground up to 15 to 20 km altitude, a combination Nd:YAG and excimer based system is used for measurements from 15 km to 55 km for ozone and 15 km to 90 km for temperature.xe2x80x9d
Finally, the NASA ATMOS observatory (http://remus.jpl.nasa.gov/atmos/) operates from orbit around the Earth, making measurements xe2x80x9cat altitudes between 10 and 150 kmxe2x80x9d. The observatory makes measurements by monitoring the absorption of solar radiation by the atmosphere during xe2x80x9cthose periods during each orbit of the spacecraft when the atmosphere is between the Sun and the instrument (i.e., at sunrise and sunset as seen from the spacecraft)xe2x80x9d.
Each of these approaches presents a set of associated difficulties. While airborne measurements do allow a single spectrometer to be used at a great number of locations, it is often impractical, if not impossible, due to political and other restrictions on airspace, to transport spectrometers to all possible regions of interest. Earth-based systems are even less versatile in that the spectrometers are typically not easily repositioned, and each unit is too costly to allow for widespread deployment of units. While the ATMOS observatory can take measurements at an expansive set of points within the atmosphere, measurements can only be made when the location of interest is in sunrise or sunset relative to the orbiting observatory. Furthermore, measurements must be made very rapidly, as xe2x80x9cthe height of the tangent point changes at about 2 kilometers per second so that, to be able to distinguish changes in the composition with altitude, successive measurements of the spectrum must be made very rapidlyxe2x80x9d (http://remus.jpl.nasa.gov/atmos/). Finally, taking measurements tangentially through the atmosphere limits the lowest elevation for which data can be obtained.
It would be advantageous to provide an atmospheric spectroscopy technique that can quickly and efficiently probe many locations within the atmosphere of the Earth. Such technique should allow for measurement of atmospheric composition above surface regions of the Earth that are not readily accessible to large and complicated surface equipment. In particular, it would be beneficial to detect the spread of chemical contamination within the atmosphere, including atmospheric regions above countries to which access is restricted. Finally, such technique should preferably be capable of operating in a passive mode, taking spectroscopic measurements using external radiation sources.
The invention is directed towards obtaining atmospheric spectroscopy measurements from an observation platform using retroreflectors. The observation platform is located above the surface of a planetary body, and at least one retroreflector is located on the surface of the planetary body. Electromagnetic radiation from a radiation source incident upon the retroreflector is reflected to a spectrometer located on the observation platform. By analyzing the received radiation, the spectrometer obtains atmospheric spectroscopy measurements for the atmospheric region through which the incident and reflected radiation pass.
In the preferred embodiment of the invention, a satellite serves as the observation platform, and is positioned at the L1 Lagrange point of the Earth-Sun system. Radiation from the Sun facilitates the spectroscopic measurements.
In an alternative, equally preferred embodiment of the invention, a satellite is positioned at the L2 Lagrange point of the Earth-Sun system. A radiation source onboard the satellite, preferably a laser, is directed toward the retroreflector.
In either geometry, the incident pathway between the radiation source and the retroreflector is essentially coincident with the reflected pathway between the retroreflector and the observation platform. In other embodiments of the invention, however, the incident and reflected pathways may be separated by a small angle, as permitted by the dispersive behavior of the retroreflector.
One or more corner mirrors, preferably an octahedral array of corner mirrors, may be used as a retroreflector. Alternatively, retroreflective materials may be used. The spectroscopic measurements may be made using radiation of various wavelengths, including visible light, infrared, microwave, radio, an x-ray. Finally, in alternative embodiments of the invention, spacecraft and aircraft serve as the observation platform.